1. Field
The present disclosure relates generally to systems and methods for converting analog signals into digital signals and in particular to a system and method for adaptively adjusting resolution of a digital channel.
2. Description of the Related Art
Analog-to-digital converters (ADCs) are devices that convert a continuous physical signal (typically voltage) to a digital number that represents the signal's amplitude. This is typically accomplished by sampling the input signal, and converting that sampled signal into a digital output.
The analog signal may be a continuous signal of voltage or current that represents a physical parameter such as temperature or pressure. Such analog signals may be provided by appropriate sensors.
Quantization is the process of converting a continuous range of values into a finite range of discrete values. In this manner, quantizing an analog signal means creating a series of discrete digital values that represent the analog signal. These discrete digital values may be represented in, for example, binary format.
The resolution of an analog-to-digital converter is determined by the number of discrete values that can be produced over a range of analog values. In some cases, the resolution of an analog-to-digital converter may be expressed in bits and referred to as bit resolution. An analog-to-digital converter with an N-bit resolution may be able to produce 2N discrete values. In other cases, the resolution of the analog-to-digital converter may be defined based on the type of measurements received as input. For example, the resolution of an analog-to-digital converter that receives an analog voltage signal as input may be the overall voltage measurement range for the analog-to-digital converter divided by the number of discrete values possible. In particular, the voltage resolution of an analog-to-digital converter may be determined by the least significant bit (LSB) voltage, which is the minimum change in voltage needed to cause a change in the digital output. As the least significant bit voltage decreases, the resolution increases. Other ADC performance related parameters include the sampling rate, accuracy, jitter, and signal to noise ratio.
Problems arise when ADCs are used to sample signals spanning large measurement ranges. For example, the voltage of a temperature sensor's output signal may vary between zero volts and five volts to represent a relatively large range of temperatures (e.g., −300 to 200 degrees Celsius). When converting the analog signal from the sensor to a digital signal using an ADC, values of the analog signal are periodically sampled. Each of these values is assigned a digital (e.g. binary) number, such as an 8-bit, 10-bit, or 12-bit binary number. In an example of an 8-bit ADC, zero volts may be assigned the digital number 00000000 and five volts is assigned digital number 11111111. In this 8-bit scheme, there are 256 possible binary numbers that can be assigned to sampled analog values.
When the voltage ranges represents a range of 500 degrees Celsius, and there are only 256 possible binary values, the sensor resolution (as determined by the digital numbers from the ADC) is approximately 1.95 degrees. If such a sensor and ADC were used to monitor and/or control the temperature of a cryogenic tank, the steps between binary values may be too large to detect a change of 1 degree Celsius from a cryogenic temperature to a non-cryogenic temperature.
One way to achieve higher sensor resolution is to use an ADC with a greater bit capacity to produce bits in the binary numbers. For example, a 10 bit or 12 bit ADC would provide many more steps along the range of possible analog values. However, such ADC converters are more expensive, and create more data to transmit and/or store. Another way to detect the one degree Celsius change of the foregoing example would be to weight the assignment of binary numbers so the steps/intervals are closer together in a range of interest (e.g., non-linear distribution of intervals) as in U.S. Pat. No. 9,276,599, entitled “Method and apparatus for non-uniform analog-to-digital conversion,” which is hereby incorporated by reference herein.
What is needed is a system and method for providing the advantages of higher resolution sampling without the cost or data overhead. The methods and systems described herein satisfy that need.